Methods for moding a leadframe in plastic integrated circuit devices

ABSTRACT

Methods for making packages and leadframes are enclosed. The package includes a die, a die pad, leads, bond wires, and an encapsulant. The lower surfaces of the die pad and leads are provided with a stepped profile by an etching step that etches partially through the thickness of a peripheral portion of the die pad, and also etches partially through the thickness of portions of the leads. Encapsulant material is applied by molding or liquid encapsulation techniques. The encapsulant material fills in beneath the recessed, substantially horizontal surfaces of the die pad and leads formed by the above-described partial etching step, and thereby prevents the die pad and leads from being pulled vertically from the package body. Other surface of the die pad and leads are not covered during the encapsulation step, but rather remain exposed at the lower surface of the package for connecting the package externally. After encapsulation, the die pad and leads are severed from the leadframe, and a completed package is cut from the leadframe. Packages may be cut from the leadframe with a punch or saw. A portion of the severed leads may be bent upwards at an oblique angle to facilitate connection of a solder interconnection to the package. The packages may be made one at a time, or a plurality of packages may be made simultaneously.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is a divisional of U.S. application Ser. No.09/176,614 entitled PLASTIC INTEGRATED CIRCUIT DEVICE PACKAGE ANDLEADFRAME HAVING PARTIALLY UNDERCUT LEADS AND DIE PAD filed Oct. 21,1998, issued as U.S. Pat. No. 6,281,568 B1 on Aug. 28, 2001.

BACKGROUND OF THE INVENTION

Integrated circuit die are conventionally enclosed in plastic packagesthat provide protection from hostile environments and enable electricalinterconnection between the integrated circuit die and printed circuitboards. The elements of such a package include a metal leadframe, anintegrated circuit die, bonding material to attach the integratedcircuit die to the leadframe, bond wires which electrically connect padson the integrated circuit die to individual leads of the leadframe, anda hard plastic encapsulant material which covers the other componentsand forms the exterior of the package.

The leadframe is the central supporting structure of such a package. Aportion of the leadframe is internal to the package, i.e., completelysurrounded by the plastic encapsulant. Portions of the leads of theleadframe extend eternally from the package and are used to connect thepackage externally.

Further background information concerning conventional plasticintegrated circuit packages and leadframes is contained in chapter 8 ofthe book Microelectronics Packaging Handbook (1989), which was edited byR. Tummala and E. Rymaszewski, and is published by Van NostrandReinhold, 114 Fifth Avenue, New York, N.Y.

A problem with conventional plastic packages is that their internalleadframes limit reduction of the size of the packages. Practitionershave attempted to reduce the size of packages by eliminating internalleadframes, as is shown in U.S. Pat. No. 4,530,152 to Roche et al andU.S. Pat. No. 5,172,214 to Casto, but these packages have numerousdisadvantages. The contacts of the package shown by Roche et al. in the'152 patent have orthogonal side surfaces. Accordingly, the packages arebelieved to be unreliable because the contacts could easily be pulledfrom the encapsulant material. The package shown by Casto in the '214patent has bent leads which extend vertically above the die pad to thetop of the die. Including such leads in a package would increasemanufacturing costs and limit reductions in the lateral size of thepackage. Accordingly, there is a need for a smaller and more reliableplastic package.

SUMMARY OF THE INVENTION

The present invention is directed toward improved plastic packages forhousing an integrated circuit die, and to leadframes and methods formaking such packages. In one embodiment of an assembly method for apackage within the present invention, Step 1 provides a metal leadframe.The leadframe includes a disposable rectangular frame. A die pad iswithin and connected to the frame. A plurality of leads extend laterallyfrom the frame toward the die pad without contacting the die pad.

The die pad of the leadframe has a rectangular perimeter. The die padhas a horizontal first surface upon which a die is placed during packageassembly. Opposite the first surface is a substantially planar centralsecond surface and a peripheral substantially planar third surface. Thethird surface is at the periphery of the second surface, and isvertically recessed from the second surface, so that the lower surfaceof die pad has a stepped profile. In a completed package, encapsulantmaterial fills in beneath the recessed third surface of the die pad, butdoes not cover the second surface of the die pad. The encapsulantmaterial beneath the third surface of the die pad prevents the die padfrom being pulled vertically from the package.

Each lead has a first surface, a second surface that is opposite thefirst surface, and a third surface that also is opposite the firstsurface and adjacent to the second surface. The second surface has arectangular or circular perimeter. The third surface is verticallyrecessed from the second surface, which results in the lower surface ofthe lead having a stepped profile. In a completed package, encapsulantmaterial fills in beneath the third surface of the lead, but does notcover the second surface of the lead. The second surface of the leadserves as a contact for connecting the package externally, as in an LCCpackage, or serves as a land for the connection of a solder ball. Theencapsulant material beneath the third surface of the lead prevents thelead from being pulled vertically from the package.

The leadframe is formed by a two-step wet etching process from a rolledmetal strip. The first etching step is a one or two sided etch thatetches through the metal strip and thereby transfers the desired overallpattern of the leadframe into the metal strip. The second etching stepis a single-sided etch that etches the periphery of the die pad andselected portions of the leads. The second step etches partially throughthe thickness of the die pad and leads, and thereby forms theabove-described, vertically recessed, planar or substantially planarthird surfaces in the die pad and the leads.

Step 2 places an integrated circuit on the upper first surface of thedie pad. Depending on the application, the area of the die may be lessthan the area of the first surface of the die pad, or may be greater inarea such that the die overhangs the peripheral sides of the die pad. Insome cases, the die also overhangs part of the length of the leads.

Step 3 electrically connects a bond wire or an equivalent conductorbetween each bonding pad of the die and a first surface of each lead.The portion of the lead to which the bond wire is connected may beplated, for example, with silver, gold, or other metals.

Step 4 applies a viscous adhesive encapsulant material onto the die andthe upward facing first surface of the leadframe. The encapsulantmaterial is then hardened. The encapsulant material covers the die, thebond wires, the first surfaces of the leads, the third surfaces of thedie pad and leads, and the side surfaces of the die pad and leads. Thesecond surfaces of the die pad and leads are not covered by encapsulantmaterial, but rather are exposed at the lower external surface of thepackage.

Step 5 plates the exposed surfaces of the leadframe, including theexposed second surfaces of the die pad and leads, with a metal, such ascopper, gold, lead-tin solder, tin, nickel, palladium, or any solderablemetal. Depending on the application and the material used for making theleadframe, Step 5 may be omitted.

Step 6 severs a completed package from the encapsulated leadframe. Inparticular, step 6 obliterates the disposable portions of the leadframeand/or severs the disposable portions of the leadframe, such as therectangular frame, from the non-disposable components of the leadframe,such as the die pad and leads. Depending on the method of encapsulationused in step 4, step 6 also may cut the encapsulant material to formperipheral sides of the package.

Step 6 severs the leads from the leadframe. The cut is made inside thedam bar. Depending on where the cut is made, an end portion of thesevered lead may extend laterally beyond the sides of the package. Step6 or a subsequent step also may include bending this protruding endportion of the severed lead up the side of the package so that the endportion of the lead is at an oblique angle to the lower external surfaceof the package and the encapsulated remainder of the lead. When thepackage is soldered to a printed circuit board, solder may be connectedto the upwardly bent end portion of the severed lead in addition to thehorizontal portion of the lead exposed at the lower external surface ofthe package to strengthen the solder connection. The lower externalsurface of the package includes: the second surface of the die pad,which is at the center of the bottom surface of the package; the secondsurfaces of the leads, and hardened encapsulant material, which formsthe remainder of the bottom surface of the package and isolates the diepad and leads from each other.

The package of the present invention has numerous advantages, and isuseful in numerous applications, including power devices and analogdevices. The package may be made small in size. For example, thepackages may be near chip size. In addition, the packages may be verythin. Packages having thickness as low as about 0.5 mm or less can befabricated according the present invention. In addition, the leads canbe placed close to the die, minimizing the length of bond wires. Theexposed second surface of the die pad can be connected by metal solderto the printed circuit board for package cooling.

Numerous variations of the leadframe, package, and assembly methoddescribed above also are described in this application, and also formpart of the present invention. For example, in one alternative assemblymethod, a leadframe is provided which allows a plurality of packages tobe constructed simultaneously.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of a method of making a package.

FIG. 2 is a plan view a leadframe used for making a package.

FIG. 3 is a cross-sectional side view of the die pad and leads of FIG. 2taken inside the dam bar along line 3—3 of FIG. 2.

FIG. 4 is a perspective view of the leadframe of FIG. 2 after die attachand encapsulation with a molded encapsulant.

FIG. 5 is a cross-sectional side view of a completed package where thepackage body was molded and a punch was used to separate the packagefrom the leadframe.

FIG. 6 is a cross-sectional view of the package of FIG. 5 afterattachment of a solder bump to the exposed portions of the lead.

FIG. 7 is a plan view of the lower external surface of the package ofFIG. 5.

FIG. 8 is a plan view of the lower external surface of an alternativepackage.

FIG. 9 is a cross-sectional view, taken inside a dam bar, of a die padand leads of a leadframe for making the package of FIG. 8.

FIG. 10 is a cross-sectional side view of the package of FIG. 8

FIG. 11 is a cross-sectional view of an alternative package where thedie extends laterally over the perimeter of the die pad and over part ofthe length of the leads.

FIG. 12 is a plan view of the lower external surface of the package ofFIG. 12 without solder interconnection balls.

FIG. 13 is a plan view of a leadframe for making the package of FIGS. 11and 12.

FIG. 14 is a flow chart of a method of making a plurality of packagessimultaneously.

FIG. 15 is a plan view of two matrixes of six leadframes etched into ametal strip.

FIG. 16 is a plan view of two matrixes of eight leadframes etched into ametal strip.

DETAILED DESCRIPTION

FIG. 1 is a flow chart of a method in accordance with the presentinvention for assembling an integrated circuit device package. FIG. 5shows an embodiment of a package, in accordance with the presentinvention, which may be formed by the method of FIG. 1.

Step 1 of FIG. 1 provides a metal leadframe. FIG. 2 is a plan view of aleadframe 20 in accordance with the present invention. For ease of view,shading is used in FIG. 2 (and the other figures) to distinguish themetal portions of leadframe 20 from empty space between the metalportions of leadframe 20.

Leadframe 20 of FIG. 2 is made of a conventional leadframe metal, suchas copper or copper alloys, plated copper, plated copper alloys, Alloy37 (37% nickel, 55% iron), or copper plated steel, depending on theapplication.

Leadframe 20 of FIG. 2 includes a peripheral rectangular tie bar 21 anda central rectangular dam bar 29. (Artisans will understand that theterms “rectangular” or “rectangle” include a square, which is arectangle with four equivalent sides.) In an alternative embodiment (notshown), such as where a plurality of leadframes 20 are etched into ametal strip (e.g., FIG. 16), tie bar 21 may be omitted, and theleadframe perimeter may be formed by a portion of the metal stripbetween adjacent leadframes. In another alternative embodiment (notshown), tie bar 21 and the portions of the leads between tie bar 21 anddam bar 29 may be omitted, so that the outer frame of the leadframe isdam bar 29.

A die pad 22 having a rectangular perimeter is connected to leadframe21. Die pad 22 is inside dam bar 29. Two connectors 28 connect die pad22 to dam bar 29 and tie bar 21 of leadframe 20. In Step 6 of FIG. 1,connectors 30 are severed from leadframe 20 inside of dam bar 29.

Eighteen leads 30 are connected to and extend laterally from tie bar 21through dam bar 29 toward a side of die pad 22 without contacting diepad 22. First end portion 34 of each lead 30 is adjacent to die pad 22.In Step 6 of FIG. 1, each lead 30 is severed between dam bar 29 andfirst end 34 of lead 30. In an alternative embodiment (not shown), leads30 may begin at dam bar 29, instead of at tie bar 21, and dam bar 29 andtie bar 21 may be connected by a plurality of symmetrically placedstrips.

The number, location and lateral paths shown in FIG. 2 for leads 30 ofleadframe 20 are exemplary only. The number, location, and lateral pathsof the leads will vary according to the application. An advantage of thepresent invention is that the leads can be designed to accommodate thenumber and location of the bonding pads of a particular integratedcircuit die.

Fourteen of the eighteen leads 30 of FIG. 2 are straight. Four leads 30include a lateral bend between dam bar 29 and die pad 22. Each of thestraight leads 30 include anchor ears 36, which project perpendicularlyfrom the lateral side of lead 30. Anchor ears 36 are approximatelyrectangular and are staggered on adjacent leads 30. In a completedpackage, anchor ears 36 engage the encapsulant material of the packageand prevent leads 30 from being pulled horizontally from the packagebody. Alternatively, throughholes or depressions in leads 30 may be usedinstead of anchor ears to engage the encapsulant material.

FIG. 3 is a cross-sectional side view of leadframe 20 inside parallelmembers of darn bar 29 along line 3—3 of FIG. 2. Die pad 22 and twoopposing leads 30 are shown in side view. The portions of leads 30 shownbegin immediately inside of dam bar 29. The lower surfaces of both diepad 22 and leads 30 include vertically recessed, horizontal orsubstantially horizontal surfaces.

Die pad 22 of FIG. 3 has a substantially planar or planar upper firstsurface 23, an opposite substantially planar or planar second surface24, and an opposite substantially planar or planar third surface 25.Orthogonal first side surface 26 is between first surface 23 and thirdsurface 25, and orthogonal second side surface 27 is between thirdsurface 25 and second surface 24. Third surface 25 is verticallyrecessed a distance “H1” from second surface 24. In other words, thirdsurface 25 is vertically between first surface 23 and second surface 24.The central portion of die pad 22 has a height “H” between first surface23 and second surface 24. Third surface 25 of die pad 22 is at theperimeter of second surface 24, and in one embodiment, surrounds secondsurface 24.

Each lead 30 of FIG. 3 includes a planar or substantially planar firstsurface 31. Opposite first surface 31 is a planar or substantiallyplanar second surface 32 and a planar or substantially planar thirdsurface 33. Second surface 32 begins at dam bar 29 and extends a shortdistance inside dam bar 29 towards die pad 22. In this embodiment,second surface 32 has a rectangular perimeter. The length of secondsurface 32 varies with the application, but should be sufficiently sizedfor external connection of the package. Third surface 33 extends betweensecond surface 32 and terminal end 34 of lead 30 adjacent to die pad 22.Third surface 33 is vertically recessed a distance “H1” from secondsurface 32. In other words, third surface 33 is vertically between firstsurface 31 and second surface 32. Anchor ears 36 (not shown) extendperpendicularly from lateral sides 37 of leads 30 adjacent to thirdsurface 33.

In Step 6 of FIG. 1, after leadframe 20 is encapsulated, leads 30 aresevered inside of dam bar 29 along rectilinear lines A—A, B—B, C—C, andD—D of FIG. 2. The cut is made vertically through the portion of lead 30which includes second surface 32. In a completed package, second surface32 of each severed lead 30 serves as a package contact to electricallyconnect the package, directly or indirectly, to a external printedcircuit board. In a completed package, third surface 33 of lead 30 iscovered with encapsulant material, and hence is internal to the packagebody (FIG. 5).

Example values for height “H” of die pad 22 and lead 30 of leadframe 20of FIG. 3 include about 0.15 to 0.50 mm, and values for “H1” includeabout 0.075 to 0.25 mm. Example values for horizontal indentation “W” ofdie pad 22 include about 0.025 to 0.25 mm. (These values also apply tothe other figures where “H,”, “H1,” and “W” are shown.) In percentageterms, the value of “H1” may be about 50%, or in the range of 33% to75%, of the value of “H.” i.e., the distance between first surfaces 23and 31 and second surfaces 24 and 32, respectively. Of course, thesevalues are examples only. Actual values depend on the application.

Leadframe 20 of FIG. 2 is formed from rolled strip metal stock by wetchemical etching. As is well known, chemical etching (also known aschemical milling) is a process that uses photolithography, photoresist,and metal-dissolving liquid chemicals to etch a pattern into a metalstrip. Typically, a layer of photoresist is applied to one or bothplanar surfaces of the strip. Next, the resist layer is exposed to lightthrough a mask having a desired pattern. The photoresist is thendeveloped and cured, forming a patterned photoresist mask; Next,chemicals are sprayed on or otherwise applied to one or both planarsurfaces of the masked strip. The exposed portions of the strip areetched away, leaving the desired pattern in the metal strip.

A two step etching process is used to form leadframe 20 of FIGS. 2 and 3(as well as FIGS. 9, 13, 15 and 16). The first etching step etches fromone or both planar surfaces of the strip according to a resist patternapplied onto one or both of the planar surfaces of the strip. This firstetching step etches completely through portions of the metal strip toform the overall pattern of the leadframe, as exemplified in FIG. 2.Next, a second resist pattern is formed on portions of one side of theleadframe. The peripheral portions of the die pad and selected portionsof the leads are not covered by the second resist pattern, and thus aresusceptible to further etching. The second etching step etches partiallythrough leadframe from one side according to the second resist pattern.This second etch step forms the recessed surfaces of leadframe 20 ofFIGS. 2 and 3, e.g., third surface 25 of die pad 22 and third surfaces33 of leads 30 inside dam bar 29. Inside dam bar 29, connectors 28typically also are subjected to this second etch step. When thechemicals have etched a selected distance through the thickness ofselected portions of the die pad and leads, the second etch step isstopped. In other words, the second etching step etches partiallythrough the thickness of selected portions of the die pad and leads. Theamount of the etching by this second etching step is governed by theneed to have a sufficient amount of encapsulant material flow beneaththird surface 25 of die pad 22 and third surfaces 33 of leads 30 tosecure die pad 22 and leads 30 to the package body. Typically, thesecond etching step removes about 50% of the thickness of the die padand leads, but the amount removed may range from about 33% to 75% of thethickness of the die pad and leads. Due to imperfections in the etchprocess, third surfaces 25 and 33 may not be planar, but rather onlysubstantially planar, and the etched sidewalls of die pad 22 and leads30 may not be at 90° angles, but rather may have radiused corners.

Alternatively, leadframe 20 may be formed by a first step of progressivestamping to form the overall pattern of the leadframe, and a second stepof chemically etching partially through the thickness of the die pad andleads of the stamped leadframe, as discussed above, to form the recessedsurfaces of leadframe 20.

Step 2 of FIG. 1 places an integrated circuit die 52 onto the center offirst surface 23 of die pad 22 (FIG. 5). The placement and attachment ofdie 52 onto die pad 22 may be performed using a conventional die attachmachine and conventional die attach epoxy. During Step 2 and thesubsequent assembly steps, leadframe 20 of FIG. 2 is grounded to protectagainst electrostatic discharge (“ESD”).

Step 3 of FIG. 1 electrically connects a conductive metal bond wire 54or equivalent between individual bonding pads 53 on integrated circuitdie 52 (FIG. 5) and first surface 31 of individual leads 30. Firstsurface 31 may be plated with gold, silver, nickel, palladium, copper orother metals. Leadframe 20 of FIG. 2 is grounded during this wiring stepto prevent damage to the integrated circuit devices due to electrostaticdischarge.

In Step 4 of FIG. 1, a viscous adhesive encapsulating material isapplied onto leadframe 20 of FIG. 2. The encapsulant material covers,among other things, integrated circuit die 52, bond wires 54, sidesurfaces 26 and 27 of die pad 22, first surface 23 and third surface 25of die pad 22, and first surface 31, third surface 33 and the sidesurfaces of leads 30 (FIGS. 4 and 5). Second surface 24 of die pad 22and second surface 32 of leads 30 are not covered with encapsulantmaterial, i.e., remain exposed. In an alternative embodiment, die pad 22may be up set during the encapsulation step so that a thin layer ofencapsulant material forms under second surface 24 of die pad 22. Insuch an embodiment, die pad 22 is entirely internal to the package body.Finally, the encapsulant material is hardened.

There are several methods by which Step 4 of FIG. 1 may be accomplished,depending on the application. For example, Step 4 of FIG. 1 may beaccomplished using conventional plastic molding techniques. In such amethod, leadframe 20 of FIG. 2 is placed in a mold, and a block of solidmolded encapsulant material is formed above and on leadframe 20, asshown in FIG. 4. The encapsulant material may be a conventional plasticmolding compound applied using conventional techniques. Example moldingcompounds include NITTO MP-8000AN molding compound from the NittoCompany of Japan, and EME 7351 UT molding compound from the SumitomoCompany of Japan. Conventional gates may be formed in leadframe 20 toassist in the molding process. The side surfaces of the mold may betapered to facilitate release from the mold.

Alternatively, instead of using a molding process for Step 4, Step 4 maybe accomplished using a liquid encapsulant. For example, as a firststep, leadframe 20 of FIG. 2 is placed on a horizontal surface. As asecond step, a contiguous bead of a conventional hardenable viscousadhesive material, such as HYSOL 4451 epoxy from the Dexter-HysolCompany of City of Industry, California, is applied onto leadframe 20,forming a closed rectangular dam around die 52 and at least the portionof leads 30 inside of dam bar 29. As a third step, the bead issolidified, such as by heating at 140° C. for one hour. As a fourthstep, a conventional hardenable viscous adhesive material suitable forencapsulating packages, such as HYSOL 4450 liquid encapsulant, isapplied within the bead so that the incomplete package within the dam iscovered with encapsulant material. As a final step, the encapsulantmaterial is hardened, such as by heating at 140° C. for one hour,forming a single solid block of encapsulant material above and onleadframe 20. Where this method is used for Step 4, Step 6 uses a saw tocut through the encapsulant material to form orthogonal package sidesand to cut a completed package from the leadframe. A similar moldingprocess and a subsequent sawing step for cutting a leadframe from such apackage is described U.S. patent application Ser. No. 09/103,760, whichwas filed on Jun. 24, 1998 and is incorporated in full herein byreference.

In Step 5 of FIG. 1, the portions of leadframe 20 of FIG. 2 which arenot covered with the encapsulant material, including second surface 24of die pad 22 and second surfaces 32 of leads 30, are plated using aconventional plating metal compatible with printed circuit boards.Example plating metals include gold, nickel palladium, inconel, lead tinsolder, or tantalum, depending on the application. Step 5 may be omittedwhere the metal used for forming leadframe 20 does not require plating,or is pre-plated. For example, Step 5 is omitted where the metal stripused for making leadframe 20 is copper with nickel palladium plating.

FIG. 4 is a perspective view of leadframe 20 of FIG. 2 after thecompletion of Steps 1-5 of FIG. 1. In this example, a molding processwas used for Step 4. A block of hardened encapsulant material formspackage body 51. The tapered sides 55 of package body 51 are within dambar 29. Accordingly, exposed portions of leads 30 extend between sides55 of package body 51 and dam bar 29.

Step 6 of FIG. 1 cuts encapsulated leadframe 20 (FIG. 4) along linesA—A, B—B, C—C, and D—D of FIG. 2. Referring to FIG. 2, Step 6 seversleads 30 inside of dam bar 29. The cut is made through second surface 32of leads 30 (FIG. 3). Step 2 also severs connectors 30 inside of dam bar29. Finally, Step 6 completes the formation of the package by cutting acompleted package away from the disposable portions of leadframe 20.

Step 6 may be performed using a punch, a saw, or equivalent shearingapparatus. For example, a punch or a saw may be used where package body35 is molded, as shown in FIG. 5. Where a punch is used, a completedpackage is cut from leadframe 20 in a single punch operation. Thepackage is inverted, and the punch cuts leads 30 inside of dam bar 29.The location of the cut can vary so that the portion of severed leads 30extending from package sides 55 can range from zero to, for example,0.50 mm in length.

FIG. 5 is a cross-sectional side view of a completed package 50 inaccordance with the present invention. Package 50 was made fromleadframe 20 of FIG. 2 and punched from FIG. 4. Package body 51 ofpackage 50 was molded. Package 50 has a planar or substantially planarexternal lower second surface 52, and tapered side surfaces 55.

Consistent with the construction of package 50 from leadframe 20 of FIG.2, die pad 22 of package 50 of FIG. 5 includes a planar or substantiallyplanar upper first surface 23. Opposite first surface 23 of die pad 22is both a planar or substantially planar second surface 24 and a planaror substantially planar peripheral third surface 25. Third surface 25surrounds second surface 22 and is vertically recessed a distance “H1”from second surface 22. Third surface 25 is vertically between firstsurface 23 and second surface 24 and is covered with the encapsulantmaterial that forms package body 51. The encapsulant material beneaththird surface 23 prevents die pad 22 from being pulled vertically fromthe package. Second surface 22 is exposed at lower surface 56 of package50, and accordingly forms part of lower second surface 56 of package 50.In alternative embodiments, die pad 22 is entirely internal to packagebody 51.

In FIG. 5, integrated circuit die 52 is on and attached to first surface23 of die pad 22. A bond wire 54 is connected between each bonding pad53 of die 52 and a first surface 31 of a lead 30.

Package 50 of FIG. 5 includes a plurality of leads 30, each of whichwere severed from leadframe 20 of FIG. 2 through a second surface 32 ata point inside of dam bar 29. The arrangement and numbers of severedleads 30 varies, depending on the design of the leadframe used to makethe package and the application. For example, as in FIG. 2, leads 30have both straight and bending lateral paths.

As in FIG. 2, each severed lead 30 includes a planar or substantiallyplanar first surface 31, an opposite planar or substantially planarsecond surface 32, and opposite planar or substantially planar thirdsurface 33. Third surface 33 is vertically recessed a distance “H1” fromsecond surface 32 so that encapsulant material covers third surface 33.In other words, third surface 33 is vertically between first surface 31and second surface 32. Second surfaces 32 of leads 30 are not covered byencapsulant material, but instead are exposed at lower surface 56 ofpackage 50.

In FIG. 5, the portion of first surface 31 of lead 30 that is internalto package body 51 is in the same horizontal plane as first surface 23of die pad 22, and third surface 33 of lead 30 is in the same horizontalplane as third surface 25 of die pad 22. In an alternative embodiment(not shown), where die pad 22 is up set in the mold, the portion offirst surface 31 of lead 30 that is inside package body 51 would be in alower horizontal plane than first surface 23 of up set die pad 22.

Each severed lead 30 of FIG. 5 includes a severed end portion 35 thatextends laterally beyond package side 55 and is bent upwards at anoblique angle θ to the horizontal remainder of second surface 32 of lead30 and lower package surface 56. Angle θ may be about 15 to 70 degrees,although the angle may vary. As shown, the upwardly bent terminalportion of second surface 32 of lead 30 is exposed. An example length ofbent end portion 35 of lead 30 is about 0.15 mm beyond package side 55,but this length may vary with the application. A range of values for thelength of end portion 35 of severed lead 30 is zero to 0.50 mm.

Severed end portion 35 of lead 30 of FIG. 5 may be bent upwards duringStep 6 by a stamping machine used to punch package 50 from leadframe 20.In an alternative embodiment (not shown), terminal portion 35 of lead 30may be bent upwards so that it is in contact with package side 55, i.e.,angle θ equals the angle from horizontal of tapered package side 55. Ina further alternative embodiment (not shown), Step 6 of FIG. 1 may cutlead 30 at package side 55 so that the severed end of lead 30 does notextend laterally beyond package side 55.

In an alternative embodiment (not shown), severed end portion 35 of lead30 extends laterally in a horizontal plane beyond package side 55. Inother words, severed end portion 35 is not bent as in FIG. 5, but ratherextends laterally in the same horizontal plane as the remainder of lead30 so that angle θ equals zero degrees. Such a package would resultwhere a saw is used for Step 6. If desired, where a saw is used for Step6, end portion 35 may be bent upwards to achieve the configuration ofFIG. 5 with the addition of a separate bending step.

In FIG. 6, a solder bump 57 is attached between package 50 and a printedcircuit board (not shown). Solder bump 57 contacts second surface 32 oflead 30 and also covers bent end portion 35 of lead 30.

In an alternative embodiment (not shown), the exposed second surface 24of die pad 22 also may be conductively connected, such as by solderpaste, to the printed circuit board to facilitate package cooling. Thecooling occurs by thermal conduction.

FIG. 7 shows the lower external surface 56 of package 50 of FIG. 5.Second surface 56 of package 50 consists of second surface 24 of die pad22, second surfaces 32 of severed leads 30, and hardened encapsulantmaterial. Second surfaces 36 of leads 30 have rectangular perimeters.Severed end portions 35 of leads 30 extend slightly beyond the edge oflower surface 56. Different sizes and shapes for second surfaces 32,such as circular, are possible depending on the application. Secondsurface 24 of die pad 22 also has a rectangular perimeter, but othershapes are possible.

In FIG. 7, second surfaces 32 of leads 30 are aligned in a row along theedges of lower surface 56 of package 50. Severed end portions 35 ofleads 30 extend slightly beyond the perimeter of lower surface 56. FIG.8 shows the lower external surface 61 of an alternative package 60,which is also within the present invention. In FIG. 8, the exposed,rectangular second surfaces 64 of severed leads 63 of FIG. 9) arealigned in single rows that are located a short distance inward from theedge of lower surface 61 of package 60. As an example, second surfaces64 are located about 0.05 to 0.50 mm from the perimeter of lower surface61 of package 60, but the distance varies with the application. In analternative embodiment (not shown), second surfaces 64 have a circular,rather than rectangular, perimeter, and form a solder interconnectionball land.

FIG. 9 is a cross-sectional view, taken inside of dam bar 29, of a diepad 22 and leads 63 of a leadframe 62 for making package 60 of FIG. 8.Leadframe 62 of FIG. 9 is largely identical to leadframe 20 of FIGS. 2and 3 and is made the same way, except as to the arrangement, number,and location of the vertically recessed lower surfaces of lead 63.Accordingly, redundant discussion is omitted.

Like lead 30 of FIG. 2, lead 63 of FIG. 9 includes a planar orsubstantially planar first surface 31 and an opposite planar orsubstantially second surface 64. Second surface 64 serves as an externalcontact for the package. Unlike second surface 32 of leadframe 20 ofFIGS. 2 and 3, however, second surface 64 of lead 63 of FIG. 9 is notlocated immediately inside and adjacent to dam bar 29 (FIG. 2), butrather is located nearer to die pad 24 between third surface 66 andfourth surface 65 of lead 63. Third surface 66 and fourth surface 65 areopposite first surface 31, are planar or substantially planar, are inthe same horizontal plane, and are vertically recessed a distance “H1 ”from second surface 64 of lead 63 (.e., are vertically between secondsurface 31 and second surface 64). Fourth surface 65 is laterallybetween dam bar 29 (not shown but similar to FIG. 2) and second surface64, and third surface 66 in between second surface 64 and die pad 22.

The perimeter of second surface 64 of lead 63 of FIGS. 8 and 9 may be avariety of shapes to facilitate different external connections of thepackage. For example, second surface 64 may have a rectangularperimeter, as in FIG. 8. Alternatively, second surface 64 may have acircular perimeter.

FIG. 10 is a cross-sectional side view of package 60 of FIG. 8. Thepackage of FIG. 10 is made according to the process of FIG. 1 using theleadframe of FIG. 9. As shown, fourth surface 65 is adjacent to packageside 57, and second surfaces 64 are located a selected distance insidethe perimeter of lower surface 61 of package 60.

In FIGS. 8 and 10, the encapsulant material forming the package bodycovers all of lead 63 except second surface 64. In other words, thirdsurface 66 and fourth surface 65 of leads 63 are covered withencapsulant material, and thus are internal to the package. Inalternative embodiments, where the severed ends of the leads extendbeyond the package sides (e.g., FIG. 5), encapsulant material also doesnot cover the portions of the severed leads that extend beyond thepackage sides.

FIG. 11 is cross sectional side view of an alternative package 70, inaccordance with the present invention, which may be made by the methodof FIG. 1. FIG. 11 is taken along line 11—11 of FIG. 12. Die 52 isattached to upper first surface 82 of die pad 72 with conventional epoxydie attach material 87. Die 52 extends over the perimeter of die pad 72and over upper first surfaces 76 of leads 73 of package 70. Accordingly,package 70 is near chip size. The distance between side 52A of die 52and package side 77 may be as little as about 0.6 mm or less on sideswhere bond wires are present. In an alternative embodiment (not shown),die 53 extends over the perimeter of die pad 72, but does not extendover leads 73. In another alternative embodiment (not shown), where bondwires are located only on two, rather than all four, sides of the die,the distance between a die side 52A where bond wires are not connectedand the package side may be as little as about 0.1 mm.

In FIG. 11, four leads 73 are shown. Only part of the length of the twoinner leads 73 are shown in this cross section because those inner leadsinclude lateral bends, as shown by leadframe 71 of FIG. 13, and theseare behind the two outside leads 73.

In FIG. 11, a short bond wire 77 is connected between each bonding pad53 on die 55 and an upper first surface 76 of a lead 73. The connectionof bond wire 77 to first surface 76 is made at a first end 86 of lead 73adjacent to package sides 79.

Package 70 of FIG. 11 is a ball grid array package, although a land gridarray(“LGA”) package also is possible. As shown in FIG. 12, an array ofsolder interconnection balls 78 is formed on lower external surface 80of package 70. Accordingly, the distances between second surfaces 74 ofdifferent leads 73 and package sides 79 vary (see FIG. 12).

Package body 81 of FIG. 11 is formed of molded encapsulant material,although other encapsulation methods may be used. During Step 4 of FIG.1, the encapsulant material fills in between lower surface 89 of die 52and first surfaces 76 of leads 73. A nonconductive (i e., insulative)adhesive epoxy 87, which is located between lower surface 89 of die 52and first surface 82 of die pad 72, attaches die 52 to die pad 72 andspaces die 52 above first surfaces 76 of leads 73. In addition, wheredie 52 extends over leads 73, additional insulative epoxy 87 is appliedbetween lower surface 89 of die 52 and first surfaces 76 of leads 73 tospace apart die 52 and leads 73.

Each lead 73 of FIG. 11 has a planar or substantially planar firstsurface 76. Opposite first surface 76 is both a planar or substantiallyplanar second surface 74 and a third surface 75. Second surface 74 islocated at a second end 85 of each lead 73 that is opposite first end86. By contrast, the locations of second surface 32 of lead 30 ofpackage 50 of FIG. 6 and second surface 64 of lead 63 of package 60 ofFIG. 8 were at or close to, respectively, the perimeter of the lowerexternal surface of their respective packages.

In FIG. 11, third surface 75 of each lead 73 is adjacent to andvertically recessed a distance “H1” from second surface 74 of lead 73.Third surface 75 is vertically between first surface 76 and secondsurface 74, and is formed by the same partial etching process as thirdsurface 33 of lead 30 of FIGS. 3 and 5, as described above. As shown,encapsulant material covers third surface 75, and thereby prevents lead73 from being pulled vertically from package body 81. Encapsulantmaterial does not cover second surface 74 of leads 73.

Die pad 72 of package 70 of FIG. 11 has a planar or substantially planarfirst surface 82. Opposite first surface 82 is both a planar orsubstantially planar second surface 83 and a peripheral planar orsubstantially planar third surface 84. Third surface 84 surrounds secondsurface 83 and is vertically recessed a distance “H1 ”*from secondsurface 83. First surface 82 of die pad 72 is in the same horizontalplane as first surface 76 of leads 73.

Third surface 84 of die pad 72 of FIG. 11 is vertically between firstsurface 82 and second surface 83 and is formed by the same partialetching process as third surface 23 of die pad 22 of FIGS. 3 and 5. Asshown in FIG. 11, encapsulant material covers third surface 84 of diepad 72, and thereby prevents die pad 72 from being pulled verticallyfrom package body 81. Encapsulant material does not cover second surface83 of die pad 72. To aid in package cooling, second surface 83 of diepad 72 may connected by solder interconnection balls or an equivalentconductor to an external printed circuit board. Alternatively, die pad72 may be up set during Step 4 of FIG. 1 so that die pad 72 is coveredby encapsulant material and therefore entirely internal to package body81. In such a case, first surface 76 of leads 73 would be below firstsurface 82 of die pad 72.

FIG. 12 is a bottom plan view of lower external surface 80 of package 70of FIG. 11 prior to the placement of solder interconnection balls onsecond surfaces 74 of leads 73. As shown, second surfaces 74 arecircular and arranged in an array. Third surfaces 75 of leads 73 are notvisible in this view because third surfaces 75 are covered withencapsulant material, and thus are internal to package body 81. A metalcorner plate 88 is at each of the four corners of lower surface 80.

FIG. 13 is a plan view of a leadframe 71 suitable for making package 70of FIGS. 11 and 12. Unlike rectangular die pad 22 of FIG. 2, die pad 72of FIG. 13 is a segmented strip connected to two parallel sides of dambar 29. Die pad 72 includes four rectangular portions 72A, which may beconnected by solder balls to a printed circuit board to facilitatepackage cooling.

Leads 73 of FIG. 13 are a variety of shapes and lengths, which varyaccording to the application. In particular, some leads 73 are laterallystraight in their extension from dam bar 29 to their respective circularsecond surfaces 74 at second lead ends 85 (FIG. 11). Other leads 73 haveone or more lateral bends between dam bar 29 and their respective secondsurfaces 74 at second lead ends 85 (FIG. 11). Two leads 73 at eachcorner of leadframe 71 are connected to the same lead end 86, but thisis not necessary. In an alternative embodiment (not shown), leads 73 mayhave anchor ears or throughholes to engage the encapsulant material.During Step 6 of FIG. 1, each lead 73 is severed from leadframe 71inside of dam bar 29 of FIG. 13. The cut is made inside dam bar 29 atthe outside edges of metal corners 88 of leadframe 71 along lines A—A,B—B, C—C, and D—D of FIG. 13.

Artisans will appreciate that numerous variations of the packages,leadframes, and assembly methods described above are possible in view ofthe present disclosure. For example, FIG. 14 is a flow chart of analternative assembly method where a plurality of packages along thelines of FIGS. 5, 10 or 11 are formed simultaneously. The basic steps ofthe FIG. 14 process are the same as the FIG. 1 process, and thus it isnot necessary for the steps to be described in detail. The differencebetween the FIG. 1 process and the FIG. 14 process is that the steps aremodified to accommodate the making of a plurality of packagessimultaneously. The process of FIG. 14 is enabled by the provision inStep 1 of a plurality of leadframes, such as leadframes 20, 62, or 71,etched adjacent to one and other in the form of a matrix in a singlesheet of metal strip.

FIG. 15 shows a matrix of twelve leadframes 71 (FIG. 11) on a metalstrip 90. The number of leadframes 71 etched into strip 90 are variable.For example, thirty six or sixty four leadframes 91 may be etched intostrip 90. Leadframes 91 were simultaneously etched into strip 90 usingthe above-described two-step chemical etching method, or the two stepprogressive stamping then chemical etching method. For the configurationof FIG. 15, Step 4 of FIG. 14 may be performed using conventionalmolding techniques, as described above, to form individual packagebodies 81 on each leadframe 71 of strip 90. In other words, the mold hasindividual mold cavities for each die, and forms an array of individualincomplete packages like FIG. 4. Step 6 cuts individual packages 70 fromstrip 90 using a punch or saw.

FIG. 16 shows an alternative strip 93 into which two matrixes of eightleadframes 20 (FIG. 2) have been etched. Instead of moldingindividualized packages during Step 4 of FIG. 14, a single block ofencapsulant material is applied over all of the leadframes 20 of each ofthe two matrixes. These blocks of encapsulant may be formed by, first,writing a bead of HYSOL 4451 adhesive around each matrix of leadframes20 of FIG. 16. After the bead is solidified, HYSOL 4450 liquidencapsulant or equivalent is applied within the bead so that each die 52and incomplete package 50 within the dam is covered with encapsulantmaterial. Next, the encapsulant material is hardened, such as byheating, forming a contiguous block of encapsulant material above and oneach of the two matrixes of leadframes 20. In Step 6 of FIG. 14, a sawis used to cut eight individual packages 50 from each of the twomatrixes of strip 93. Step 6 severs the connections between theleadframe 20 and die pad 22 and leads 30. Step 6 also cuts through theblock of encapsulant material to form orthogonal package sides.

The above description of embodiments of this invention is intended to beillustrative and not limiting. Other embodiments of this invention willbe obvious to those skilled in the art in view of the above disclosure.

What is claimed is:
 1. A method of making a package for an integratedcircuit die comprising: providing a leadframe including a disposablemetal frame, a metal die pad at a center of said frame and integrallyconnected to the frame, and a plurality of leads each having an innerend facing the die pad and an opposite outer end integrally connected tothe frame, said die pad having a horizontal first surface, a horizontalsecond surface opposite the first surface, a horizontal peripheral thirdsurface surrounding the second surface and vertically recessed from thesecond surface, and a vertical outer peripheral surface between thefirst surface and the third surface, and said leads each having ahorizontal first surface, a horizontal second surface opposite the firstsurface, and a horizontal third surface, said third surface beinglaterally adjacent to and vertically recessed from the second surfaceand opposite another portion of the first surface of the lead; placingan integrated circuit die on the first surface of the die pad;electrically connecting the integrated circuit die to the first surfaceof each lead; applying an encapsulant material so that the integratedcircuit die and the first surface and the third surface of the die padand the leads are covered by the encapsulant material, but the secondsurface of the leads is exposed; and severing the die pad and the leadsfrom the frame so as to detach the package from the frame, wherein thesecond surface of each lead is exposed in a horizontal plane of a firstexterior surface of the package.
 2. The method of claim 1, wherein theencapsulant material is applied so that the second surface of the diepad is exposed at the first exterior surface of the package.
 3. Themethod of claim 1, further comprising plating the exposed second surfaceof the leads with a metal after applying the encapsulant material. 4.The method of claim 1, wherein the leads are severed so that a firstportion of each severed lead extends outside of the encapsulantmaterial, and further comprising bending the first portion of the leadupwards relative to the first exterior surface of the package body. 5.The method of claim 1, wherein the second surface of the lead iscircular.
 6. The method of claim 1, wherein at least some of said leadshave a horizontal fourth surface laterally adjacent to and verticallyrecessed from the second surface and opposite another portion of thefirst surface of the lead, said second surface being laterally betweenthe fourth surface and the third surface.
 7. The method of 1, whereinthe integrated circuit die overhangs the outer peripheral surface of thedie pad.
 8. The method of 7, wherein the integrated circuit die extendsover the first surface of a plurality of the leads.
 9. The method of 8,further comprising applying a first insulative material over the firstsurface of the leads; and wherein the integrated circuit die is placedso that the first insulative material is between the first surface ofthe leads and the integrated circuit die prior to the step of applyingthe encapsulating material.
 10. The method of claim 1, wherein the leadsare arranged so that the exposed second surfaces of the leads of thepackage form an array with a plurality of rows and a plurality ofcolumns.
 11. The method of claim 10, wherein at least some of the leadsinclude a lateral bend.
 12. The method of claim 1, wherein the thirdsurface of the die pad was formed by etching a partial distance throughthe die pad from the second surface toward the first surface of the diepad, and the third surface of the leads was formed by etching a partialdistance through the lead from the second surface toward the firstsurface of the lead.
 13. A method of making a package for an integratedcircuit die comprising; providing a leadframe including a disposablemetal frame, a metal die pad at a center of the frame and integrallyconnected to the frame, and a plurality of leads each having an innerend facing the die pad and an opposite outer end integrally connected tothe frame, wherein said die pad has a horizontal first surface, ahorizontal second surface opposite the first surface, a horizontalperipheral third surface surrounding the second surface and verticallyrecessed from the second surface, and a vertical outer peripheralsurface between the first surface and the third surface, said thirdsurface being having been formed by etching a partial distance from thesecond surface toward the first surface, and wherein said leads eachhave a first surface and a second surface opposite the first surface;placing an integrated circuit die on the first surface of the die pad;electrically connecting the integrated circuit die to the first surfaceof each lead; applying an encapsulant material so that the integratedcircuit die and the first surface and the third surface of the die padand the leads are covered by the encapsulant material, but the secondsurface of each of the leads is exposed; and severing the die pad andthe leads from the frame so as to detach the package from the frame,wherein the second surface of each lead is exposed in a horizontal planeof the first exterior surface of the package.
 14. The method of claim13, wherein the encapsulant material is applied so that the secondsurface of the die pad is exposed at the first exterior surface of thepackage.
 15. The method of claim 13, wherein the integrated circuit dieoverhangs the outer peripheral surface of the die pad.
 16. The method ofclaim 13, wherein the integrated circuit die extends over the firstsurface of a plurality of the leads.
 17. The method of claim 13, whereinat least some of the leads include a lateral bend.
 18. The method o fclaim 17, wherein the integrated circuit device extends over the firstsurface of a plurality of the leads.
 19. The method of claim 18, whereinthe leads are arranged so that the exposed second surfaces of the leadsof the package form an array with a plurality of rows and a plurality ofcolumns.
 20. The method of claim 13, wherein the leads are arranged sothat the exposed second surfaces of the leads of the package form anarray with a plurality of rows and a plurality of columns.
 21. Themethod of claim 13, further comprising applying a first insulativematerial over the first surface of the leads; and placing the integratedcircuit die so that the first insulative material is between the firstsurface of the leads and the integrated circuit die prior to the step ofapplying the encapsulating material.